The present invention relates to a temperature sensor for measuring a substrate temperature during thermal processing.
In rapid thermal processing (RTP), a substrate is heated quickly and uniformly to a high temperature, such as 400xc2x0 Celsius (C.) or more, to perform a fabrication step such as annealing, cleaning, chemical vapor deposition, oxidation, or nitration. For example, a thermal processing system, such as the RTP tool available from Applied Materials, Inc., under the trade name xe2x80x9cCentura(copyright)xe2x80x9d, may be used to perform metal annealing at temperatures of 400xc2x0 C. to 500xc2x0 C., titanium silicide formation at temperatures around 650xc2x0 C., or oxidation or implant annealing at temperatures around 1000xc2x0 C.
The temperature of the substrate must be precisely controlled during these thermal processing steps to obtain high yields and process reliability, particularly given the submicron dimension of current semiconductor devices. For example, to fabricate a dielectric layer 60-80 angstroms (xc3x85) thick with a uniformity of +/xe2x88x922 xc3x85, a typical requirement in current device structures, the temperature in successive processing runs cannot vary by more than a few xc2x0C. from the target temperature. To achieve this level of temperature control, the temperature of the substrate is measured in real time and in situ.
Optical pyrometry is a technology that is used to measure substrate temperatures in RTP systems. Pyrometry exploits a general property of objects, namely, that objects emit radiation with a particular spectral content and intensity that is characteristic of their temperature. Thus, by measuring the emitted radiation, the object""s temperature can be determined. A pyrometer measures the emitted radiation intensity and performs the appropriate conversion to obtain the substrate temperature. The relationship between spectral intensity and temperature depends on the spectral emissivity of the substrate and the ideal blackbody intensity-temperature relationship, given by Planck""s law:                                           I            b                    ⁡                      (                          λ              ,              T                        )                          =                              2            ⁢                          C              1                                                          λ              5                        ⁡                          (                                                ⅇ                                                            c                      2                                                              λ                      ⁢                                              xe2x80x83                                            ⁢                      T                                                                      -                            )                                                          (        1        )            
where C1 and C2 are known constants, xcex is the radiation wavelength of interest, and T is the substrate temperature measured in xc2x0K. The spectral emissivity xcex5(xcex,T) of an object is the ratio of its emitted spectral intensity I(xcex,T) to that of a black body at the same temperature IB(xcex,T). That is,                               ϵ          ⁡                      (                          λ              ,              T                        )                          =                              I            ⁡                          (                              λ                ,                T                            )                                                          I              b                        ⁡                          (                              λ                ,                T                            )                                                          (        2        )            
Since C1 and C2 are known constants, under ideal conditions, the temperature of the substrate can be accurately determined if xcex5(xcex,T) is known.
The emissivity of a substrate depends on many factors, including the characteristics of the wafer itself (e.g., temperature, surface roughness, doping level of various impurities, material composition and thickness of surface layers), the characteristics of the process chamber, and the process history of the wafer. Therefore, a priori estimation of substrate emissivity cannot provide a general purpose pyrometric temperature measurement capability. Consequently, the emissivity of the substrate needs to be measured in situ. Furthermore, any uncertainty in the measured emissivity introduces an uncertainty into the temperature measurement.
To reduce this uncertainty, several techniques have been developed for reducing the effect of substrate emissivity on the temperature measurement. One such technique involves placing a reflector plate beneath the back surface of a target substrate to form a reflecting cavity. If the reflector plate were an ideal reflector, it can be shown that because all of the radiation emitted from the substrate would be reflected back to the substrate, the reflecting cavity would act as an ideal black body. That is, the intensity of the radiation within the reflecting cavity would not be a function of the emissivity of the surface of the substrate. Thus, in the ideal case, the reflecting cavity increases the effective emissivity of the substrate to a value equal to one.
However, because the reflector plate is not an ideal reflector, the effective emissivity of the substrate will be less than one, although it will be higher than the substrate""s actual emissivity. Therefore, the radiation intensity measured by a temperature sensor will still depend upon the emissivity of the substrate. Consequently, although variations in the actual emissivity of the substrate will have less impact on the measured temperature, there will be uncertainty in the temperature measurement.
Moreover, different portions of the substrate may have different emissivities. Consequently, if the emissivity of the substrate is measured in only one region, there will be uncertainty in the temperature measurements of other regions of the substrate.
In addition, the transparency of the substrate contributes to the uncertainty in the temperature measurement. A portion of the radiation intended to heat the substrate may instead pass through the substrate into the pyrometer. Since the pyrometer includes this transmitted radiation in the measured intensity, the transmitted radiation results in an artificially high temperature measurement. Assuming that the substrate is a silicon wafer and the pyrometer is sensitive to infrared radiation, this problem will be more acute at lower processing temperatures (e.g., less than 600xc2x0 C.), where the transmitivity of silicon to infrared radiation is higher.
Another source of uncertainty is electrical noise. If only a small amount of light enters the pyrometer, the signal-to-noise ratio will be low, and the electrical noise will create uncertainty in the temperature measurement.
Another problem is that there are many thermal processing steps which are not compatible with a highly reflective reflector plate. For example, certain thermal processing steps may be corrosive or destructive to such a reflector plate.
Furthermore, even standard thermal processing may cause the reflector plate to become dirty or corroded over time, and thus less reflective. If the reflector plate""s reflectivity decreases, the substrate""s effective emissivity also decreases. This change in the substrate""s effective emissivity changes the intensity of the radiation sampled by the pyrometer, and can create an error in the measured temperature.
In one aspect, the invention is directed to a temperature sensor for measuring a temperature of a substrate in a thermal processing chamber, where the chamber includes a reflector forming a reflecting cavity with a substrate when the substrate is positioned in the chamber. The temperature sensor includes a probe having an input end positioned to receive radiation from the reflecting cavity, and a detector optically coupled to an output end of the probe. The radiation entering the probe includes reflected radiation and non-reflected radiation. The detector measures an intensity of a first portion of the radiation entering the probe to generate a first intensity signal and measures an intensity of a second portion of the radiation entering the probe to generate a second intensity signal. The detector is configured so that a ratio of the reflected radiation to the non-reflected radiation is higher in the first portion than the second portion.
Implementations of the invention may include the following. A processor may be coupled to the detector to calculate a substrate temperature and a substrate emissivity from the first and second intensity signals. The second portion of radiation may include a greater proportion of radiation which enters the probe with an axis of propagation within an angle, e.g., between 0 and 10 degrees, of an axis normal to the reflector than the first portion of radiation. The detector may include a first detector surface and a second detector surface, and the first portion of the radiation may impinge the first detector surface and the second portion of the radiation may impinge the second detector surface.
The detector can be configured to preferentially direct radiation that enters the probe with an axis of propagation within an angle of an axis normal to the reflector to the second detector surface, to preferentially direct radiation that enters the probe with an axis of propagation outside an angle of an axis normal to the reflector to the first detector surface, to prevent a portion of the radiation that enters the probe with an axis of propagation within an angle of an axis normal to the reflector from impinging on the first detector surface, or to prevent a portion of the radiation that enters the probe with an axis of propagation outside an angle of an axis normal to the reflector from impinging on the second detector surface.
The detector may include a reflective surface positioned to divide the radiation from the probe into a first beam that is directed to the first detector surface and a second beam that is directed to the second detector surface. A pyrometer filter may be positioned in an optical path between the probe and the reflective surface. The reflective surface may be positioned in a central region of an optical path of the radiation entering the probe. The reflective surface may be partially reflective and partially transparent, and the radiation reflected by the reflective surface may form one of the first and second beams and radiation transmitted by the reflective surface may form the other of the first and second beams. The detector may include an opaque optical element positioned in the outer region of the second beam, or an opaque optical element positioned in the inner portion of the first beam.
The detector may include an array of photosensitive elements, and radiation from a central region of the optical path may impinge a central region of the array and radiation from the outer region of the optical path may impinge an outer region of the array. The detector may include circuitry configured to use signals from the photosensitive elements located in the outer region to generate the first intensity signal and signals from the photosensitive elements located in the central region to generate the second intensity signal. The detector may include a first detector surface positioned to receive the central portion of the radiation and a second detector surface positioned to receive the outer portion of the radiation. For example, the first detector surface may be substantially annular in shape and surround the second detector surface. The detector may include a split optical fiber having a first branch and a second branch, the split optical fiber configured so that the outer portion of the radiation enters the first branch to form a first beam and the central portion of the radiation enters the second branch to form a second beam.
In another aspect, the invention is directed to an apparatus for measuring the temperature of a substrate in a thermal processing chamber. The apparatus includes a reflector located to form a reflecting cavity with a substrate when the substrate is positioned in the chamber, a temperature sensor including a probe and a detector to generate first and second intensity signals, and a processor coupled to the detector to calculate a substrate temperature from the first and second intensity signals. The probe has an input end positioned to receive radiation from the reflecting cavity, and an output end optically coupled to the detector. The detector measures an intensity of a first portion of the radiation entering the probe to generate the first intensity signal and measures an intensity of a second portion of the radiation entering the probe to generate the second intensity signal. The radiation entering the probe includes reflected radiation and non-reflected radiation, and the temperature sensor is configured so that a ratio of the reflected radiation to the non-reflected radiation is higher in the first portion than the second portion.
In another aspect, the invention is directed to an apparatus for measuring the temperature of a substrate in a thermal processing chamber. The apparatus includes a reflector located to form a reflecting cavity with a substrate when the substrate is positioned in the chamber, a temperature sensor including a probe having an input end positioned to receive radiation from the reflecting cavity, means for directing a first portion of the radiation from an output end of the probe to a first detector and directing a second portion of the radiation from the output end of the probe to a second detector, and a processor coupled to the first and second detectors to calculate a substrate temperature from a first intensity signal from the first detector and a second intensity signal from the second detector. The radiation entering the probe including reflected radiation and non-reflected radiation, and the directing means is configured so that a ratio of reflected radiation to non-reflected radiation is higher in the first portion than the second portion.
In another aspect, the invention is directed to an apparatus including a reflective collimator having an input aperture to receive radiation from a probe, a reflective concentrator having an input aperture positioned to receive radiation from an output aperture of the reflective collimator, a filter positioned between the output aperture of the reflective collimator and the input aperture of the reflective concentrator, a first detector positioned to receive radiation from an output aperture of the reflective concentrator and generate a first intensity signal, a second detector to generate a second intensity signal, and a reflective surface positioned in the optical path of the radiation passing through the reflective concentrator to direct a portion of the radiation to the second detector.
Implementations of the invention may include the following. A processor may be connected to the first and second detectors to determine a temperature measurement from the first and second intensity signals. Either or both the collimator and the concentrator may be a xcex8in/xcex8out device.
In another aspect, the invention is directed to a method of measuring the temperature of a substrate in a thermal processing chamber. In the method, a substrate is positioned in a thermal processing chamber to form a reflecting cavity with a reflector located in the chamber. Radiation is sampled from the cavity with a probe and the sampled radiation is directed to a detector. The sampled radiation includes reflected and non-reflected radiation. A first intensity signal is generated for a first portion of the sampled radiation with the detector, and a second intensity signal is generated for a second portion of the sampled radiation with the detector, wherein a ratio of the non-reflected radiation to the reflected radiation is higher in the first portion than the second portion. A temperature of the substrate is determined from the first and second intensity signals.
Implementations of the invention may include the following. An emissivity of the substrate may be determined from the first and second intensity signals. The radiation may be divided into a first beam which is directed to a first detector surface and a second beam which is directed to a second detector surface.
Advantages of the invention may include the following. The temperature may be calculated using the measured emissivity to generate a more accurate temperature measurement. Furthermore, the emissivity measurements may be performed without introducing additional probes into the processing chamber. The actual emissivity of the substrate has less effect on the temperature measured by the temperature sensor. The emissivity of the substrate may be measured at multiple locations. The effect of radiation transmitted through the substrate is reduced, and the signal-to-noise ration is increased, thereby decreasing the uncertainty in the temperature measurement. A partially reflective (e.g., as low as 50% reflectivity) reflector plate may be used to create a virtual black body cavity. This permits the reflector plate to be made from less expensive materials. It also permits the reflector plate to be made of materials which are more compatible with the more destructive or corrosive thermal processes.
Other features and advantages of the invention will be apparent from the following description, including the drawings and the claims.